Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, laptop computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention. Among them, the development of rechargeable secondary batteries has been the focus of particular interest. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Among currently available secondary batteries, lithium ion secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than traditional batteries using aqueous electrolyte solutions, such as Ni-MH batteries, Ni—Cd batteries, H2SO4—Pb batteries, and the like. However, such lithium secondary batteries have disadvantages of safety-related problems caused by the use of organic electrolyte solutions, for example, ignition and explosion, and complex manufacturing.
Such electrochemical devices are produced by many companies, but their safety characteristics show different aspects from each other. Assessing and ensuring the safety of electrochemical devices is important. One of the most important considerations is that electrochemical devices should not cause damage to users in the event of malfunction, and for this purpose, Safety Standards impose strict regulations on ignition and explosion of electrochemical devices. In the safety characteristics of electrochemical devices, electrochemical devices have a high risk of explosion in the event of overheat or thermal runaway of an electrochemical device or penetration of a separator.
Particularly, a polyolefin-based separator commonly used as a separator of an electrochemical device has a drawback of thermal contraction to an original size at high temperature due to characteristics of a separator material, for example, characteristics of polyolefins that generally melt at temperature equal to or less than 200° C., and processing characteristics, for example, characteristics that pass through a stretching process to adjust the pore size and porosity. Thus, when a battery increases in temperature by internal/external stimulation, there is a high likelihood for a short circuit between a cathode and an anode due to shrinkage or melting of a separator, and as a result, the battery has a high risk of explosion due to emission of electrical energy.
Accordingly, to solve the problem of a polyolefin-based separator, there is a need for technology development for a material capable of improving performance and safety of an electrochemical device as well as serving as a separator.